Heat transfer is the net passage of energy as a result of temperature differences. This energy is transferred in the direction of decreasing temperature until thermal equilibrium (equality of temperatures) is achieved. The basic mechanisms involved in this process include radiation (the transfer of energy in the form of electromagnetic waves) and conduction (the transfer of kinetic energy). Heat transfer in fluids can occur at a faster rate, because large masses of a fluid can be displaced and can mix with other fluid masses of different temperatures. This process is considered a distinct mechanism called convection. In many heat transfer processes, radiation and convection or conduction work together, although one is often dominant.

Every object emits electromagnetic radiation in a wave spectrum related to its own temperature. An object cooler than its surroundings will absorb more energy in the form of radiation than it emits. This radiation can...